1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to a method for providing Hybrid Automatic Repeat Request (HARQ) in a high-speed packet data communication system.
2. Description of the Related Art
Typically, when there is a need for a mobile communication system to perform high-speed data transmission, the mobile communication system adapts a Hybrid Automatic Repeat Request (HARQ) scheme to increase transmission efficiency (i.e., transmission throughput). Particularly, the HARQ scheme has been frequently adapted to a specific channel (e.g., a forward channel for a mobile communication system) in which channel status is abruptly changed and other service traffic channels are contained. With the increasing development of high-speed data transmission services, many developers have conducted intensive research into new mobile communication technologies for adapting a new HARQ scheme using a variable code rate Error Correction Code (ECC) instead of a conventional HARQ scheme using a fixed code rate ECC.
Code Division Multiple Access 2000 (CDMA2000) First Evolution Data and Voice (1x EV-DV) system adapts two channels to perform packet data transmission, i.e., an Forward Packet Data Channel (F-PDCH) for use in payload traffic, and an Forward Packet Data Control Channel (F-PDCCH) for controlling the F-PDCH. The F-PDCH is a channel for transmitting an Encoder Packet (EP) serving as a transmission data block, and a maximum of 2 channels may be available for the F-PDCH, such that the F-PDCH can simultaneously transmit individual EPs to two terminals using a Code Division Multiplexing (CDM) scheme.
A method for transmitting EPs over the above F-PDCH will hereinafter be described in detail.
Each EP is encoded by a turbo encoder, and is divided into 4 subpackets having different Increment Redundancy IR patterns using a Quasi-Complementary Turbo Code (QCTC) symbol selection process. The subpacket functions as a basic unit for an initial transmission mode or a re-transmission mode. For the initial transmission or re-transmission mode, IR patterns of individual subpackets are distinguished from each other according an Subpacket Identifier (SPID). A subpacket-based modulation scheme (e.g., Quaternary Phase Shift Keying (QPSK), 8 Phase Shift Keying (PSK), or 16 Quadrature Amplitude Modulation (QAM) and a transmission slot length (e.g., 1, 2, or 4 slots) are determined according to forward channel quality information of a terminal and a variety of resource conditions (e.g., the number of Walsh codes allocatable to the F-PDCH and power information, etc.) of a Base Station (BS).
F-PDCCH having information associated with demodulation and decoding of the F-PDCH is multiplexed along with the F-PDCH over an orthogonal channel different from that of the F-PDCH in the same slot period as in the F-PDCH, and is then transmitted to a mobile station (MS). The information associated with demodulation and decoding of the F-PDCH contains ARQ Channel ID (ACID) information for classifying ARQ channels, EP-SIZE information for indicating an EP's bit size, and EP-NEW information for discriminating between two successive EPs contained in the same ARQ channel.
A method for receiving transmission packet data and packet data control information will hereinafter be described in detail.
The Moblie Station (MS) decodes the F-PDCCH, and determines if the transmission packet is its own packet. If it is determined that the transmission packet is the MS's packet, the MS demodulates and decodes the F-PDCH. If the current reception subpacket is a predetermined packet created by re-transmission of a previously-received EP, the MS performs code combining with code symbols of the previously-received EP in such a way that a decoding operation is performed. If the decoding process is successfully performed, the MS transmits an Acknowledgement (ACK) signal over an Reverse—ACK/NAK transmission Channel (R-ACKCH) to command the BS to transmit the next EP. Otherwise, if the decoding process is unsuccessful, the MS transmits the NAK signal to command the BS to re-transmit the same EP. A physical layer HARQ operation unit associated with a single EP is typically called an ARQ channel. The CDMA2000 1x EV-DV standard has disclosed that a maximum of 4 ARQ channel operations are made available at one time, and this is denoted by “N=4 Fast HARQ channel”.
The CDMA2000 1x EV-DV standard controls the MS to inform the BS of a predetermined ACK/NAK time delay and the number of ARQ channels available at one time, and this is considered to be the most important implementation issue of the MS. The ACK/NAK time delay is needed for the MS to perform a packet reception operation and transmit the ACK/NAK signal. The ACK/NAK time delay supported by the MS may be one slot (i.e., 1-slot) of 1.25 msec or two slots (i.e., 2-slots). The number of ARQ channels may be 2, 3, or 4.
The CDMA2000 1x EV-DV standard has adapted a QCTC scheme as an encoding scheme available for a high-speed HARQ. The QCTC scheme provides a variable code rate, and guarantees the improvement of soft-combining operation created by the HARQ. According to the CDMA2000 1x EV-DV standard, packet data transmission/reception is established by a physical layer HARQ operation structure. This physical layer HARQ operation structure indicates that some parts of all HARQ operations move to a physical layer such that it can acquire a high response rate and a high processing rate, whereas an ACK/NAK response associated with a conventional data re-transmission process and its associated HARQ operation have been performed in an upper layer. Considering function and role aspects, the physical layer HARQ may be contained in a MUX layer existing in a Media Access Control (MAC) layer. The physical layer determines if data re-transmission is performed, such that a processing time consumed for the same data can be shortened. If the upper layer transmits the NAK signal, it is impossible for the upper layer to perform soft-combining of the same data. However, if the physical layer transmits a NAK signal, it can perform soft-combining of code symbols associated with the same EP, resulting in increased use efficiency of channel resources. Preferably, the HARQ protocol is transferred to a predetermined place under the MUX layer contained in the MAC layer such that the physical layer can perform the HARQ operation. The physical layer HARQ implemented under the MUX layer is also called a Fast HARQ. A conventional RLP-based ARQ control scheme creates a considerable round trip delay of a minimum of 200 msec for a predetermined period of time during which an NAK signal is received after performing one-packet transmission and then a re-transmission packet associated with the NAK signal is transmitted. Contrary to the above RLP-based ARQ control scheme, the physical layer HARQ control scheme creates a very low round trip delay of a minimum of a few milliseconds.
As stated above, provided that the conventional HARQ control operation for use in the upper layer is performed under the MUX layer, a high-speed HARQ response and process needed for high-speed data transmission is made available. However, this solution is merely a logical solution available in the range of a standard specification, such that it has the following disadvantages in real cases.
Most mobile communication systems currently implement the upper layer having the MUX layer using a software program stored in a Central Processing Unit (CPU), however, the CPU for use in the MS does not have a good processing speed and capacity. Therefore, if the HARQ protocol requesting such a rapid response is implemented with the MS's CPU, overload unexpectedly occurs in the CPU's clocks, such that the MS may be operate incorrectly.
N independent HARQ controllers and N independent turbo decoders are needed to support an N-channel HARQ. The number of HARQ controllers and the number of turbo decoders increase in proportion to the value of ‘N’, resulting in increased power-consumption and complexity of the MS. In conclusion, there must be implemented new technology capable of supporting the N-channel HARQ using a minimum number of HARQ controllers and a minimum number of turbo decoders.